Dichroic polarizing film and optical polarizer containing the film

ABSTRACT

A dichroic polarizing film is made, for example, by, first combining polyvinyl alcohol and a second polymer, such as, polyvinyl pyrrolidone or a sulfonated polyester, in a solvent. The ratio of polyvinyl alcohol to second polymer is between about 5:1 to 100:1 by weight. The film is coated on a substrate, dried, and then stretched to orient at least a portion of the film. The film incorporates a dichroic dye material, such as iodine, to form a dichroic polarizer. This polarizer may be used in conjunction with a multilayer optical film, such as a reflective polarizer, to form an optical polarizer. The multilayer optical film may contain two or more sets of polyester films, where at least one of the sets is birefringent and orientable by stretching. The polyvinyl alcohol/second polymer film and the multilayer optical film may be simultaneously stretched to orient both polymer films.

This is a continuation of application Ser. No. 09/604,491 filed Jun. 27,2000 now U.S. Pat. No. 6,335,051 which is a continuation of applicationSer. No. 09/006,458 filed Jan. 13, 1998, now U.S. Pat. No. 6,113,811

FIELD OF THE INVENTION

The present invention relates to a polarizing film and an opticalpolarizer containing the film. More particularly, the invention relatesto a dichroic polarizing film made from a dispersion or solution of apolyvinyl alcohol and a second polymer, and an optical polarizercontaining the film.

BACKGROUND OF THE INVENTION

Optical polarizing film is widely used for glare reduction and forincreasing optical contrast in such products as sunglasses and LiquidCrystal Displays (LCD). One of the most commonly used types ofpolarizers for these applications is a dichroic polarizer which absorbslight of one polarization and transmits light of the other polarization.One type of dichroic polarizer is made by incorporating a dye into apolymer matrix which is stretched in at least one direction. Dichroicpolarizers may also be made by uniaxially stretching a polymer matrixand staining the matrix with a dichroic dye. Alternatively, a polymermatrix may be stained with an oriented dichroic dye. Dichroic dyesinclude anthraquinone and azo dyes, as well as iodine. Many commercialdichroic polarizers use polyvinyl alcohol as the polymer matrix for thedye.

Another type of polarizer is a reflective polarizer which reflects lightof one polarization and transmits light of another orthogonalpolarization. One type of reflective polarizer is made by forming astack of alternating sets of polymer layers, one of the sets beingbirefringent to form reflective interfaces in the stack. Typically, theindices of refraction of the layers in the two sets are approximatelyequal in one direction so that light polarized in a plane parallel tothat direction is transmitted. The indices of refraction are typicallydifferent in a second, orthogonal direction so that light polarized in aplane parallel to the orthogonal direction is reflected.

One measure of performance for polarizers is the extinction ratio. Theextinction ratio is the ratio of a) light transmitted by the polarizerin a preferentially transmitted polarization state to b) lighttransmitted in an orthogonal polarization state. These two orthogonalstates are often related to the two linear polarizations of light.However, other types of orthogonal states, such as, left andright-handed circular polarizations or two orthogonal ellipticalpolarizations may also be used. The extinction ratios of dichroicpolarizers vary over a wide range depending on their specificconstruction and target application. For example, dichroic polarizersmay have extinction ratios between 5:1 and 3000:1. Dichroic polarizersused in display systems typically have extinction ratios which arepreferably greater than 100:1 and even more preferably greater than500:1.

Dichroic polarizers typically absorb light in the non-transmissionpolarization. However, dichroic polarizers also absorb some of the lighthaving the high transmission polarization. The amount of this absorptiondepends on the details of the construction of the polarizer and thedesigned extinction ratio. For high performance display polarizers, suchas those used in LCDs, this absorption loss is typically between about 5and 15%. The reflectivity of these polarizers for light having theabsorption (i.e., low transmission) polarization tends to be small. Evenwith surface reflections included, this reflectivity is typically lessthan 10% and usually less than 5%.

Reflective polarizers typically reflect light having one polarizationand transmit light having an orthogonal polarization. Reflectivepolarizers often have incomplete reflectivity of the high extinctionpolarization over a wavelength region of interest. Typically, thereflectivity is greater than 50% and is often greater than 90% or 95%. Areflective polarizer will also typically have some absorption of lighthaving the high transmission polarization. Typically, this absorption isless than about 5 to 15%.

The above two types of polarizers may be combined to make a singleoptical polarizer, thereby incorporating the useful characteristics ofboth types of polarizers. These polarizers may be formed and,optionally, oriented together. Unfortunately, the polyvinyl alcohol filmused in many dichroic polarizers tends to crack under the processingconditions necessary to prepare some reflective polarizers, including,for example, those which use polyethylene naphthalate (PEN) or coPENoptical layers. These reflective polarizers may be formed by stretchinga polymeric film at processing temperatures, such as 135 to 180° C., anda stretch ratio of between 2:1 and 10:1. There is a need for a dichroicfilm layer that does not crack under these processing conditions.

Dichroic polarizers may also be used with other optical devices, such asother types of reflective polarizers and mirrors. The combination of adichroic polarizer with an IR mirror may be useful for reducing glare.The formation of the dichroic polarizer in combination with the mirrorretains the processing difficulties mentioned above, especially when themirror is made using oriented polyester layers.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to dichroic polarizing filmsand their use in optical polarizers. In one embodiment, a polarizingfilm includes a polymeric film which contains polyvinyl alcohol and asecond polymer. The polymeric film is oriented and incorporates adichroic dye material. The dichroic dye material may be incorporatedbefore or after stretching of the film.

Another embodiment is an optical device which includes a substrate and apolarizing film. The polarizing film is disposed on the substrate andcontains polyvinyl alcohol and a second polymer. The polymeric film isoriented and incorporates a dichroic dye material.

A further embodiment is a method of making an optical device. The methodincludes forming a dispersion of polyvinyl alcohol and a second polymerin a solvent. A substrate is coated with the dispersion/solution andthen the solvent is removed from the dispersion to form a polymericfilm. The polymeric film is then oriented by stretching in at least onedirection. A dichroic dye material is also incorporated in the polymericfilm.

Another embodiment is a display device made from a polarizing film. Thepolarizing film includes a polymeric film which contains polyvinylalcohol and a second polymer. The polymeric film is oriented andincorporates a dichroic dye material.

The above summary of the invention is not intended to describe eachillustrated embodiment or every implementation of the present invention.The figures and the detailed description which follow more particularlyexemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view of one embodiment of an opticalpolarizer according to the present invention;

FIG. 2 is a side elevational view of one embodiment of a multilayeroptical film for use in the optical polarizer of FIG. 1; and

FIG. 3 is a side elevational view of another embodiment of a multilayeroptical film for use in the optical polarizer of FIG. 1.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The invention is directed to optical polarizers and in particular todichroic polarizers. The invention is also directed to the formation ofthese polarizers and to their use with other optical elements, such asreflective polarizers, mirrors, and IR mirrors.,

Some conventional dichroic polarizers 11 are made using polyvinylalcohol films. These films are well-known in the art and have been used,upon incorporation of a dye material, as dichroic polarizers. Tofunction as a diebroic polarizer, the polyvinyl alcohol film istypically stretched to orient the film. When stained, the orientation ofthe film determines the optical properties (e.g., the axis ofextinction) of the film. One use of these films is in conjunction withmultilayer optical films which are also stretched to orient one or morelayers of the film to form, for example, reflective polarizers andmirrors. Examples of such multilayer optical films may be found in U.S.patent application Ser. Nos. 08/402,041, 09/006,601, 09/006,288, and09/006,455, all of which are incorporated herein by reference. Othermultilayer optical films, reflective polarizers, mirrors, and opticaldevices may also be used in conjunction with the dichroic polarizers.

Unfortunately, polyvinyl alcohol films tend to crack under thestretching conditions used in the formation of many reflectivepolarizers, including, for example, those made from multilayer polyesterfilms, and in particular, polyester films containing naphthalatesubunits such as PEN. Although no particular theory is necessary to theinvention, it is thought that polyvinyl alcohol forms a hydrogen-bondednetwork which is incapable of stretching under these conditions whilemaintaining its structural integrity. The hydrogen-bonded network isstrained and, finally, at one or more points slips, thereby causingcracks. Experimentation indicates that small molecule plasticizers donot solve this problem.

It has been found that the addition of a second polymer dispersible orsoluble in a solvent used in the formation of the polyvinyl alcohol filmsignificantly reduces cracking. The second polymer is included as eithera dispersion or a solution, depending on the nature of the secondpolymer, and the terms “dispersion” and “solution” will be usedinterchangeably herein. The second polymer is preferably water-solubleas water is a common solvent for polyvinyl alcohol. More preferably, thesecond polymer is a polar polymer. Suitable second polymers include, forexample, polyvinyl pyrrolidone and polyesters dispersible in the solventof the polyvinyl alcohol. Examples of water-soluble or water dispersiblepolyesters include sulfonated polyesters, such as those describe in U.S.Pat. No. 5,427,835, incorporated herein by reference. Suitableco-solvents include, for example, polar solvents such as C1-C4 alcohols.

Typically, the polyvinyl alcohol and second polymer are mixed in a ratioof between 5:1 and 100:1 by weight, and preferably between 8:1 and 20:1by weight. The solution is typically 1 to 50 wt. % solids, andpreferably 5 to 25 wt. % solids. Although no particular theory isnecessary to the invention, it is thought that the addition of thesecond polymer separates the hydrogen-bonded network into a large numberof domains which may move relative to each other when strained, therebyrelieving the strain and reducing the amount of cracking.

The polyvinyl alcohol film may be made by a variety of techniques. Oneexemplary method for making the film includes combining the polyvinylalcohol and the second polymer in a solvent according to theabove-mentioned ratios and weight percentages. This dispersion/solutionof the two polymers is then applied to the surface of a substrate. Thesubstrate may be another film, a multilayer stack, a plastic object, orany other surface which allows stretching of the polyvinyl alcohol film.Application of the dispersion/solution may be accomplished by a varietyof known methods, including, for example, coating the substrate usingtechniques, such as shoe coating, extrusion coating, roll coating,curtain coating, or any other coating method capable of providing auniform coating. The substrate may be coated with a primer or treatedwith a corona discharge to help anchor the polyvinyl alcohol film to thesubstrate. Typically, the thickness of the coating is 25 to 500 μm whenwet and preferably 50 to 125 μm. After coating, the polyvinyl alcoholfilm is dried at a temperature typically between 100° C. and 150° C. Thefilm is then stretched using, for example, length orienters or tenterclips to orient the film. In some embodiments, the film is removed fromthe substrate. The film may then be adhered to another surface, ifdesired. The polyvinyl alcohol film, when stained, can then be used as adichroic polarizer. However, it will be understood that other uses maybe made of the polyvinyl alcohol film.

A finished polyvinyl alcohol film typically includes a dichroic dyematerial to form a dichroic polarizer. The dichroic dye material mayinclude dyes, pigments, and the like. Suitable dye materials for use inthe dichroic polarizer film include, for example, iodine, as well asanthraquinone and azo dyes, such as Congo Red (sodiumdiphenyl-bis-α-naphthylamine sulfonate), methylene blue, stilbene dye(Color Index (CI)=620), and 1,1′-diethyl-2,2′-cyanine chloride (CI=374(orange) or CI=518 (blue)). The properties of these dyes, and methods ofmaking them, are described in E. H. Land, Colloid Chemistry (1946).Still other dichroic dyes, and methods of making them, are discussed inthe Kirk Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661(4th Ed. 1993), and in the references cited therein.

The dichroic dye material may be added to the dispersion or solution ofthe polyvinyl alcohol and second polymer prior to coating.Alternatively, a polyvinyl alcohol film may be stained with a stainingcomposition, such as, for example, an iodine-containing solution. Thestaining of the polyvinyl alcohol film may occur before or after thefilm is drawn. In some cases, the dichroic dye material may not be ableto withstand the drawing conditions and should therefore be applied tothe polyvinyl alcohol film after drawing.

One example of a suitable staining composition is an iodine-containingsolution. The iodine stained film may be stabilized using, for example,a boroncontaining composition, such as a boric acid/borax solution.Other stains may require different stabilizers. The concentrations ofthe staining or stabilizing compositions, as well as the temperature atwhich the staining or stabilization occurs and the time of contact witheach solution, may vary widely without compromising the stain.

Various other components may be added to the dispersion/solution ofpolyvinyl alcohol and the second polymer. For example, a surfactant maybe added to facilitate wetting of the substrate. A wide variety ofsurfactants may be used, including, for example, Triton X-100 (UnionCarbide Chemicals and Plastics Company, Inc., Danbury, Conn.). Thesurfactant is typically about 1% or less of the solution, and preferablyabout 0.5% or less. The surfactant is preferably nonionic so that itdoes not interfere with polar groups on the polymer.

Another optional additive is a drying aid which facilitates filmformation on drying. Example of a suitable drying aids includesN-methyl-pyrrolidone and butyl carbitol. The drying aid is typicallyabout 10% or less of the solution, and preferably about 5% or less.

Various functional layers or coatings may be added to the optical filmsand devices of the present invention to alter or improve their physicalor chemical properties, particularly along the surface of the film ordevice. Such layers or coatings may include, for example, slip agents,low adhesion backside materials, conductive layers, antistatic coatingsor films, barrier layers, flame retardants, UV stabilizers, abrasionresistant materials, optical coatings, compensation films, retardationfilms, diffuse adhesives, and/or substrates designed to improve themechanical integrity or strength of the film or device. In addition, anadhesive may be applied to the polyvinyl alcohol film to adhere the filmto the substrate. This may be particularly useful when the polyvinylalcohol film is removed from a first substrate and then placed on asecond substrate. A variety of adhesives may be used including, forexample, resins and pressure sensitive adhesives (PSA). When choosing asuitable adhesive, the optical properties of the adhesive are usuallyconsidered. The addition of a second polymer to the polyvinyl alcoholfilm provides an improved dichroic polarizer which is compatible withthe simultaneous orientation of the polyvinyl alcohol film and amultilayer optical film, such as a reflective polarizer or mirror film.The advantage of using the improved dichroic polarizer is that thedichroic and multilayer optical film may be oriented together, therebyforming, for example, an optical polarizer which may have dichroic andreflective elements that are more perfectly aligned. Furthermore, theaddition of a second polymer to the polyvinyl alcohol film oftenimproves the adhesion of the film to a substrate.

An exemplary process for forming optical devices includes, first,forming a multilayer optical film, as described below. This multilayeroptical film is coated or laminated with a polyvinyl alcohol film whichincorporates the second polymer. This may be accomplished usingwell-known devices, such as, for example, shoe coating, extrusioncoating, roll coating, curtain coating, or any other coating methodcapable of providing a uniform coating.

The multilayer optical film and the polyvinyl alcohol film are thensimultaneously drawn to form an oriented multilayer optical film and anoriented polyvinyl alcohol film. In some embodiments, the multilayeroptical film is drawn multiple times. In these embodiments, thepolyvinyl alcohol film is often coated or laminated on the multilayeroptical film prior to the final draw. In alternative embodiments, thetwo films may be drawn and oriented separately. Known devices may beused to draw the two films, including, for example, tenters or longorienters. Drawing the polyvinyl alcohol film and the multilayer opticalfilm together typically results in the orientation axis of the polyvinylalcohol layer being coincident with the axis of final orientation of themultilayer optical film, which may be either a polarizer film or amirror film. Dichroic dye material may be added prior to drawing thefilms, or may be incorporated later by, for example, staining thepolyvinyl alcohol film, as described above.

A number of different combinations of dichroic polarizer and multilayerpolymer films may be formed. For example, a visible band dichroic andreflective polarizer combination, an IR band mirror and dichroicpolarizer combination, an IR band polarizer and dichroic polarizercombination, among others, may be formed.

FIG. 1 illustrates an exemplary device, namely an optical polarizer 10which includes a dichroic polarizer 11 and a reflective polarizer 12.This combination of two different types of polarizers may create anoptical polarizer with a high reflection/absorption of light of onepolarization and a high transmission of light with a second, orthogonalpolarization. Typically, the two polarizers are aligned with respect toeach other to provide maximum transmissivity of light having aparticular polarization.

The dichroic polarizer 11 is typically in close proximity to thereflective polarizer 12, although this is not necessary. Preferably, thetwo polarizers 11, 12 are bonded to each other to eliminate any air gap.

The reflective polarizer 12 usually reflects a substantial portion oflight having a first polarization and transmits most of the light havinga second, orthogonal polarization. The dichroic polarizer 11 typicallyabsorbs most of light having a third polarization and transmits asubstantial portion of light having a fourth, orthogonal polarization.Often, the optical polarizer 10 is formed by orienting the reflectivepolarizer 12 and the dichroic polarizer 11 so that they transmit lightof a particular polarization (i.e., the second and fourth polarizationare the same) and reflect/absorb light of an orthogonal polarization(i.e., the first and third polarizations are the same). The presentinvention will be discussed with reference to this particularconfiguration. However, other configurations in which the reflectivepolarizer 12 and the dichroic polarizer 11 are oriented in a differentmanner with respect to each other are also possible and included withinthe invention.

In use, the combined polarizers are illuminated on one or both of theoutside facing surfaces, as illustrated in FIG. 1. Ray 13 is lighthaving a polarization that is preferentially reflected by the reflectivepolarizer 12 to form ray 14. Ray 15 is light from ray 13 that istransmitted by the reflective polarizer 12. Typically, ray 15 is muchless intense than ray 14. In addition, ray 15 is usually attenuated bythe dichroic polarizer 11. Light ray 16, which is orthogonally polarizedto ray 13, is preferentially transmitted by the reflective polarizer 12and is typically only slightly attenuated by the dichroic polarizer 11.

Ray 17 is light having a polarization that is preferentially absorbed bythe dichroic polarizer 11, and which preferably has the samepolarization as ray 13. The portion of ray 17 which is transmitted bythe dichroic polarizer 11 is further attenuated by reflection off thereflective polarizer 12, thereby forming ray 18. Light ray 19 ispolarized orthogonally to ray 17 and preferably has the samepolarization as ray 16. Ray 19 is preferentially transmitted by both thedichroic polarizer 11 and the reflective polarizer 12.

Combining the dichroic polarizer 11 with the reflective polarizer 12results in an optical polarizer 10 which has a higher extinction ratioof the transmitted light than would be the case with a dichroicpolarizer by itself. This allows for the use of a dichroic polarizerwith a lower extinction ratio. This may be useful, as dichroicpolarizers typically absorb some of the light that is to be transmitted.Using a dichroic polarizer with a lower extinction ratio may increasethe amount of light of the desired polarization which is transmitted.For light polarized along the extinction axis, the preferred extinctionpercentage for the dichroic polarizer is 10% or greater, more preferredis 55% or greater, and most preferred is 70% or greater. The best choiceof dichroic and reflective polarizers depends on the design goals,including the allowed reflectivity from the dichroic polarizer side ofthe film, the extinction ratio of the reflective polarizer, and thedesired final polarizer contrast.

The combination of the reflective polarizer with the dichroic polarizerhas other advantages. For example, this combination has a highreflectivity from one side of the film for one polarization and a lowreflectivity from the other side of the film. The combination of thesetwo characteristics may be useful in a number of systems includingdirect view LCDs. For example, a direct view LCD might have a back sidereflectivity of 1% and require a final extinction ratio greater than1000:1. To achieve 1% reflectivity when combined with a reflectivepolarizer with an approximately 100% reflectivity, the dichroicpolarizer would need to transmit 10% or less of light polarized in theextinction direction. If the reflective polarizer has an extinctionratio of 50:1 then the dichroic polarizer would typically require anextinction ratio of at least 20:1 to achieve the final extinction ratioof 1000:1.

The reflective polarizer 12 may contain internal structure, such asinterfaces between different materials, where the index is not exactlymatched in the appropriate directions, or other scattering centers. Bothof these types of internal structure may interfere with light whichwould normally be transmitted through the polarizer. In general, it ispreferred that the reflection of light having the transmissionpolarization by the reflective polarizer 12 be about 30% or less, morepreferably about 20% or less, and most preferably about 15% or less. Inaddition, the reflectivity of the reflective polarizer depends on thewavelength range and the angle of incident light. The preferredreflection percentage by the reflective polarizer 12 for light havingthe reflection polarization and within a wavelength range of interest is20% or greater, more preferably 50% or greater and most preferably 90%or greater.

Similar design features and parameters may be used when the multilayeroptical film is a mirror or IR mirror. The preferred reflectionpercentage by a mirror for light with a wavelength range of interest,whether visible or infrared, is 20% or greater, more preferably about50% or greater, and most preferably about 90% or greater.

One example of a useful multilayer optical film 20 is shown in FIG. 2.This multilayer optical film 20 may be used to make reflectivepolarizers, mirrors and other optical devices. The multilayer opticalfilm 20 includes one or more first optical layers 22, one or more secondoptical layers 24, and one or more non-optical layers 28. The firstoptical layers 22 are often birefringent polymer layers which areuniaxially- or biaxially-oriented. In some embodiments, the firstoptical layers 22 are not birefringent. The second optical layers 24 maybe polymer layers which are birefringent and uniaxially- orbiaxially-oriented. More typically, however, the second optical layers24 have an isotropic index of refraction which is different than atleast one of the indices of refraction of the first optical layers 22after orientation. The methods of manufacture and use, as well as designconsiderations for the multilayer optical films 20 are described indetail in U.S. patent application Ser. Nos. 08/402,041, 09/006,601, and09/006,288, all of which are herein incorporated by reference. Although,the present invention will be primarily exemplified by multilayeroptical films 20 with second optical layers 24 which have an isotropicindex of refraction, the principles and examples described herein may beapplied to multilayer optical films 20 with second optical layers 24that are birefringent, as described in, for example, U.S. patentapplication Ser. No. 09/006,455, which is also herein incorporated byreference.

Additional sets of optical layers, similar to the first and secondoptical layers 22, 24, may also be used in the multilayer optical film20. The design principles disclosed herein for the sets of first andsecond optical layers may be applied to any additional sets of opticallayers. Furthermore, it will be appreciated that, although only a singlestack 26 is illustrated in FIG. 2, the multilayer optical film 20 may bemade from multiple stacks that are subsequently combined to form thefilm 20.

The optical layers 22, 24 and, optionally, one or more of thenon-optical layers 28 are typically placed one on top of the other toform a stack 26 of layers. Usually the optical layers 22, 24 arearranged as alternating pairs, as shown in FIG. 2, to form a series ofinterfaces between layers with different optical properties. The opticallayers 22, 24 are typically less than 1 μm thick, although thickerlayers may be used. Furthermore, although FIG. 2 shows only six opticallayers 22, 24, many multilayer optical films 20 have a large number ofoptical layers. Typical multilayer optical films 20 have about 2 to 5000optical layers, preferably about 25 to 2000 optical layers, morepreferably about 50 to 1500 optical layers, and most preferably about 75to 1000 optical layers.

The non-optical layers 28 are polymer films that are disposed within(see FIG. 3) and/or over (see FIG. 2) the stack 26 to protect theoptical layers 22, 24 from damage, to aid in the co-extrusionprocessing, and/or to enhance post-processing mechanical properties. Thenon-optical layers 28 are often thicker than the optical layers 22, 24.The thickness of the non-optical layers 28 is usually at least twotimes, preferably at least four times, and more preferably at least tentimes, the thickness of the individual optical layers 22, 24. Thethickness of the non-optical layers 28 may be varied to obtain aparticular thickness of the optical film 20. Typically, one or more ofthe non-optical layers 28 are placed so that at least a portion of thelight to be transmitted, polarized, and/or reflected by the opticallayers 22, 24, also travels through the non-optical layers (i.e., thenon-optical layers are placed in the path of light which travels throughor is reflected by the optical layers 22, 24).

As a non-limiting example, the optical layers 22, 24 and the non-opticallayers 28 of the multilayer optical film 20 may be made using polymers,such as polyesters. The term “polymer” includes polymers and copolymers,as well as polymers and/or copolymers which may be formed in a miscibleblend, for example, by coextrusion or by reactions, including, forexample, transesterification. Polyesters have carboxylate and glycolsubunits which are generated by reactions of carboxylate monomermolecules with glycol monomer molecules. Each carboxylate monomermolecule has two or more carboxylic acid or ester functional groups andeach glycol monomer molecule has two or more hydroxy functional groups.The carboxylate monomer molecules may all be the same or there may betwo or more different types of molecules. The same applies to the glycolmonomer molecules.

The properties of a polymer layer or film vary with the particularchoice of monomer molecules. One example of a polyester useful inmultilayer optical films is polyethylene naphthalate (PEN) which can bemade, for example, by reactions of naphthalene dicarboxylic acid withethylene glycol.

Suitable carboxylate monomer molecules for use in forming thecarboxylate subunits of the polyester layers include, for example,2,6-naphthalene dicarboxylic acid and isomers thereof; tereplithalicacid; isophthalic acid; phthalic acid; azelaic acid; adipic acid;sebacic acid; norbornene dicarboxylic acid; bi-cyclooctane dicarboxylicacid; 1,6-cyclohexane dicarboxylic acid and isomers thereof; t-butylisophthalic acid, tri-mellitic acid, sodium sulfonated isophthalic acid;2,2′-biphenyl dicarboxylic acid and isomers thereof, and lower alkylesters of these acids, such as methyl or ethyl esters. The term “loweralkyl” refers, in this context, to C1-C10 straight-chained or branchedalkyl groups. Also included within the term “polyester” arepolycarbonates which are derived from the reaction of glycol monomermolecules with esters of carbonic acid.

Suitable glycol monomer molecules for use in forming glycol subunits ofthe polyester layers include ethylene glycol; propylene glycol;1,4-butanediol and isomers thereof; 1,6-hexanediol; neopentyl glycol;polyethylene glycol; diethylene glycol; tricyclodecanediol;1,4-cyclohexanedimethanol and isomers thereof, norbornanediol;bicyclo-octanediol; trimethylol propane; pentaerythritol;1,4-benzenedimethanol and isomers thereof; bisphenol A; 1,8-dihydroxybiphenyl and isomers thereof; and 1,3-bis(2-hydroxyethoxy)benzene.

Non-polyester polymers are also useful in creating polarizer or mirrorfilms. For example, layers made from a polyester such as polyethylenenaphthalate may be combined with layers made from an acrylic polymer toform a highly reflective mirror film. In addition, polyether imides mayalso be used with polyesters, such as PEN and coPEN, to generate amultilayer optical film 20. Other polyester/non-polyester combinations,such as polybutylene terephthalate and polyvinyl chloride, may also beused.

Multilayered optical films may also be made using only non-polyesters.For example, poly(methyl methacrylate) and polyvinylidene fluoride maybe used to make layers for a multilayer optical film 20. Anothernon-polyester combination is atactic or syndiotactic polystyrene andpolyphenylene oxide. Other combinations may also be used.

The first optical layers 22 are typically orientable polymer films, suchas polyester films, which may be made birefringent by, for example,stretching the first optical layers 22 in a desired direction ordirections. The term “birefringent” means that the indices of refractionin orthogonal x, y, and z directions are not all the same. For films orlayers in a film, a convenient choice of x, y, and z axes is shown inFIG. 2 in which the x and y axes correspond to the length and width ofthe film or layer and the z axis corresponds to the thickness of thelayer or film. In the embodiment illustrated in FIG. 2, the multilayeroptical film 20 has several optical layers 22, 24 which are stacked oneon top of the other in the z-direction.

The first optical layers 22 may be uniaxially-oriented, for example, bystretching in a single direction. A second orthogonal direction may beallowed to neck into some value less than its original length. In oneembodiment, the direction of stretching substantially corresponds toeither the x or y axis shown in FIG. 2. However, other directions may bechosen. A birefringent, uniaxially-oriented layer typically exhibits adifference between the transmission and/or reflection of incident lightrays having a plane of polarization parallel to the oriented direction(i.e., stretch direction) and light rays having a plane of polarizationparallel to a transverse direction (i.e., a direction orthogonal to thestretch direction). For example, when an orientable polyester film isstretched along the x axis, the typical result is that n_(x)≠n_(y),where n_(x)and n_(y)are the indices of refraction for light polarized ina plane parallel to the “x” and “y” axes, respectively. The degree ofalteration in the index of refraction along the stretch direction willdepend on factors such as the amount of stretching, the stretch rate,the temperature of the film during stretching, the thickness of thefilm, the variation in film thickness, and the composition of the film.Typically, the first optical layers 22 have an in-plane birefringence(the absolute value of n_(x)−n_(y)) after orientation of 0.04 or greaterat 632.8 nm, preferably about 0.1 or greater, and more preferably about0.2 or greater. All birefringence and index of refraction values arereported for 632.8 nm light unless otherwise indicated.

Polyethylene naphthalate (PEN) is an example of a useful material forforming the first optical layers 22 because it is highly birefringentafter stretching. The refractive index of PEN for 632.8 nm lightpolarized in a plane parallel to the stretch direction increases fromabout 1.62 to as high as about 1.87. Within the visible spectrum, PENexhibits a birefringence of 0.20 to 0.40 over a wavelength range of400-700 nm for a typical high orientation stretch (e.g., a materialstretched to five or more times its original dimension at a temperatureof 130° C. and an initial strain rate of 20%/min).

The birefringence of a material can be increased by increasing themolecular orientation. Many birefringent materials are crystalline orsemicrystalline. The term “crystalline” will be used herein to refer toboth crystalline and semicrystalline materials. PEN and othercrystalline polyesters, such as polybutylene naphthalate (PBN),polyethylene terephthalate (PET) and polybutylene terephthalate (PDT),are examples of crystalline materials useful in the construction ofbirefringent film layers such as is often the case for the first opticallayers 22. In addition, some copolymers of PEN, PBN, PET, and PBT arealso crystalline or semicrystalline. The addition of a comonomer to PEN,PBN, PET, or PBT may enhance other properties of the material including,for example, adhesion to the second optical layers 24 or the non-opticallayers 28 and/or the lowering of the working temperature (i.e., thetemperature for extrusion and/or stretching the film).

If the polyester material of the first optical layers 22 contains morethan one type of carboxylate subunit, then the polyester may be a blockcopolyester to enhance adhesion to other layers (e.g., the secondoptical layers 24 or non-optical layers 28) made from block copolymershaving similar blocks. Random copolyesters may also be used.

Referring again to FIGS. 2 and 3, one or more of the non-optical layers28 may be formed as a skin layer over at least one surface of stack 26as illustrated in FIG. 2, to, for example, protect the optical layers22, 24 from physical damage during processing and/or afterwards. Inaddition, one or more of non-optical layers 28 may be formed within thestack 26 of layers, as illustrated in FIG. 3, to, for example, providegreater mechanical strength to the stack or to protect the stack duringprocessing.

The non-optical layers 28 ideally do not significantly participate inthe determination of optical properties of the multilayer optical film20, at least across the wavelength region of interest. The non-opticallayers 28 are typically not birefringent or orientable but in some casesthis may not be true. Typically, when the non-optical layers 28 are usedas skin layers there will be at least some surface reflection. If amultilayer optical film 20 is to be a reflective polarizer, thenon-optical layers preferably have an index of refraction which isrelatively low. This decreases the amount of surface reflection. If themultilayer optical film 20 is to be a mirror, the non-optical layers 28preferably have an index of refraction which is high, to increase thereflection of light.

When the non-optical layers 28 are found within the stack 26, there willtypically be at least some polarization or reflection of light by thenon-optical layers 28 in combination with the optical layers 22, 24adjacent to the non-optical layers 28. Typically, however, thenon-optical layers 28 have a thickness which dictates that lightreflected by the non-optical layers 28 within the stack 26 has awavelength outside the region of interest, for example, in the infraredregion for visible light polarizers or mirrors.

The non-optical layers 28 may also be made from copolyesters similar tothe second optical layers 24, using similar materials and similaramounts of each material. In addition, other polymers may also be used,as described above with respect to the second optical layers 24. It hasbeen found that the use of coPEN (i.e., a copolymer of PEN) or othercopolymer material for skin layers (as seen in FIG. 2) reduces thesplittiness (i.e., the breaking apart of a film due to strain-inducedcrystallinity and alignment of a majority of the polymer molecules inthe direction of orientation) of the multilayer optical film 20, becausethe coPEN of the skin layers orients very little when stretched underthe conditions used to orient the first optical layers 22.

Preferably, the polyesters of the first optical layers 22, the secondoptical layers 24, and the non-optical layers 28 are chosen to havesimilar rheological properties (e.g., melt viscosities) so that they canbe co-extruded. Typically, the second optical layers 24 and thenon-optical layers 28 have a glass transition temperature, T_(g), thatis either below or no greater than about 40° C. above the glasstransition temperature of the first optical layers 22. Preferably, theglass transition temperature of the second optical layers 24 and thenon-optical layers 28 is below the glass transition temperature of thefirst optical layers 22.

A reflective polarizer may be made by combining a uniaxially-orientedfirst optical layer 22 with a second optical layer 24 having anisotropic index of refraction that is approximately equal to one of thein-plane indices of the oriented layer. Alternatively, both opticallayers 12,14 are formed from birefringent polymers and are oriented in amultiple draw process so that the indices of refraction in a singlein-plane direction are approximately equal. The interface between thetwo optical layers 12,14, in either case, forms a light reflectionplane. Light polarized in a plane parallel to the direction in which theindices of refraction of the two layers are approximately equal will besubstantially transmitted. Light polarized in a plane parallel to thedirection in which the two layers have different indices will be atleast partially reflected. The reflectivity can be increased byincreasing the number of layers or by increasing the difference in theindices of refraction between the first and second layers 22, 24.

Typically, the highest reflectivity for a particular interface occurs ata wavelength corresponding to twice the combined optical thickness ofthe pair of optical layers 22, 24 which form the interface. The opticalthickness of the two layers is n₁d₁+n₂d₂ where n₁, n₂ are the indices ofrefraction of the two layers and d₁, d₂ the thicknesses of the layers.The layers 22, 24 may each be a quarter wavelength thick or the layers22, 24 may have different optical thicknesses, so long as the sum of theoptical thicknesses is half of a wavelength (or a multiple thereof). Afilm having a plurality of layers may include layers with differentoptical thicknesses to increase the reflectivity of the film over arange of wavelengths. For example, a film may include pairs of layerswhich are individually tuned to achieve optimal reflection of lighthaving particular wavelengths.

Alternatively, the first optical layers 22 may be biaxially-oriented bystretching in two different directions. The stretching of optical layers22 in the two directions may result in a net symmetrical or asymmetricalstretch in the two chosen orthogonal axes.

One example of the formation of a mirror using a multilayer optical film20 is the combination of a biaxially-oriented optical layer 22 with asecond optical layer 24 having indices of refraction which differ fromboth the in-plane indices of the biaxially-oriented layer. The mirroroperates by reflecting light having either polarization because of theindex of refraction mismatch between the two optical layers 22, 24.Mirrors may also be made using a combination of uniaxially-orientedlayers with in-plane indices of refraction which differ significantly.In another embodiment, the first optical layers 22 are not birefringentand a mirror is formed by combining first and second optical layers 22,24 which have significantly different indices of refraction. Reflectionoccurs without orientation of the layers. There are other methods andcombinations of layers that are known for producing both mirrors andpolarizers and which may be used. Those particular combinationsdiscussed above are merely exemplary.

The second optical layers 24 may be prepared with a variety of opticalproperties depending, at least in part, on the desired operation of themultilayer optical film 20. In one embodiment, the second optical layers24 are made of a polymer material that does not appreciably opticallyorient when stretched under conditions which are used to orient thefirst optical layers 22. Such layers are particularly useful in theformation of reflective polarizing films, because they allow theformation of a stack 26 of layers by, for example, coextrusion, whichcan then be stretched to orient the first optical layers 22 while thesecond optical layers 24 remain relatively isotropic. Typically, theindex of refraction of the second optical layers 24 is approximatelyequal to one of the indices of the oriented first optical layers 22 toallow transmission of light with a polarization in a plane parallel tothe direction of the matched indices. Preferably, the two approximatelyequal indices of refraction differ by about 0.05 or less, and morepreferably by about 0.02 or less, at 632.8 nm. In one embodiment, theindex of refraction of the second optical layers 24 is approximatelyequal to the index of refraction of the first optical layers 22 prior tostretching.

In other embodiments, the second optical layers 24 are orientable. Insome cases, the second optical layers 24 have one in-plane index ofrefraction that is substantially the same as the corresponding index ofrefraction of the first optical layers 22 after orientation of the twosets of layers 22, 24, while the other in-plane index of refraction issubstantially different than that of the first optical layers 22. Inother cases, particularly for mirror applications, both in-plane indicesof refraction of the optical layers 22, 24 are substantially differentafter orientation.

A brief description of one method for forming multilayer polymer filmsis described. A fuller description of the process conditions andconsiderations is found in U.S. patent application Ser. No. 09/006,288,incorporated herein by reference. The multilayer polymer films areformed by extrusion of polymers to be used in the first and secondoptical layers, as well as the non-optical layers. Extrusion conditionsare chosen to adequately feed, melt, mix and pump the polymer resin feedstreams in a continuous and stable manner. Final melt streamtemperatures are chosen to be within a range which reduces freezing,crystallization or unduly high pressure drops at the low end of therange and which reduces degradation at the high end of the range. Theentire melt stream processing of more than one polymer, up to andincluding film casting on a chill roll, is often referred to asco-extrusion.

Following extrusion, each melt stream is conveyed through a neck tubeinto a gear pump used to regulate the continuous and uniform rate ofpolymer flow. A static mixing unit may be placed at the end of the necktube to carry the polymer melt stream from the gear pump into amultilayer feedblock with uniform melt stream temperature. The entiremelt stream is typically heated as uniformly as possible to enhance bothuniform flow of the melt stream and reduce degradation during meltprocessing.

Multilayer feedblocks divide each of two or more polymer melt streamsinto many layers, interleave these layers, and combine the many layersinto a single multilayer stream. The layers from any given melt streamare created by sequentially bleeding off part of the stream from a mainflow channel into side channel tubes which lead to layer slots in thefeed block manifold. The layer flow is often controlled by choices madein machinery, as well as the shape and physical dimensions of theindividual side channel tubes and layer slots.

The side channel tubes and layer slots of the two or more melt streamsare often interleaved to, for example, form alternating layers. Thefeedblock's downstream-side manifold is often shaped to compress anduniformly spread the layers of the combined multilayer stacktransversely. Thick, non-optical layers, known as protective boundarylayers (PBLs), may be fed near the manifold walls using the melt streamsof the optical multilayer stack, or by a separate melt stream. Asdescribed above, these non-optical layers may be used to protect thethinner optical layers from the effects of wall stress and possibleresulting flow instabilities.

The multilayer stack exiting the feedblock manifold may then enter afinal shaping unit such as a die. Alternatively, the stream may besplit, preferably normal to the layers in the stack, to form two or moremultilayer streams that may be recombined by stacking. The stream mayalso be split at an angle other than normal to the layers. A flowchanneling system that splits and stacks the streams is called amultiplier. The width of the split streams (i.e., the sum of thethicknesses of the individual layers) can be equal or unequal. Themultiplier ratio is defined as the ratio of the wider to narrower streamwidths. Unequal streams widths (i.e., multiplier ratios greater thanunity) can be useful in creating layer thickness gradients. In the caseof unequal stream widths, the multiplier may spread the narrower streamand/or compress the wider stream transversely to the thickness and flowdirections to ensure matching layer widths upon stacking.

Prior to multiplication, additional non-optical layers can be added tothe multilayer stack. These non-optical layers may perform as PBLswithin the multiplier. After multiplication and stacking, some of theselayers may form internal boundary layers between optical layers, whileothers form skin layers.

After multiplication, the web is directed to the final shaping unit. Theweb is then cast onto a chill roll, sometimes also referred to as acasting wheel or casting drum. This casting is often assisted byelectrostatic pinning, the details of which are well-known in the art ofpolymer film manufacture. The web may be cast to a uniform thicknessacross the web or a deliberate profiling of the web thickness may beinduced using die lip controls.

The multilayer web is then drawn to produce the final multilayer opticalfilm. In one exemplary method for making a multilayer optical polarizer,a single drawing step is used. This process may be performed in a tenteror a length orienter. Typical tenters draw transversely (TD) to the webpath, although certain tenters are equipped with mechanisms to draw orrelax (shrink) the film dimensionally in the web path or machinedirection (MD). Thus, in this exemplary method, a film is drawn in onein-plane direction. The second in-plane dimension is either heldconstant as in a conventional tenter, or is allowed to neck in to asmaller width as in a length orienter. Such necking in may besubstantial and increase with draw ratio.

In one exemplary method for making a multilayer mirror, a two stepdrawing process is used to orient the birefringent material in bothin-plane directions. The draw processes may be any combination of thesingle step processes described that allow drawing in two in-planedirections. In addition, a tenter that allows drawing along MD, e.g. abiaxial tenter which can draw in two directions sequentially orsimultaneously, may be used. In this latter case, a single biaxial drawprocess may be used.

In still another method for making a multilayer polarizer, a multipledrawing process is used that exploits the different behavior of thevarious materials to the individual drawing steps to make the differentlayers comprising the different materials within a single coextrudedmultilayer film possess different degrees and types of orientationrelative to each other. Mirrors can also be formed in this manner.

The following examples demonstrate the manufacture and uses of theinvention. It is to be understood that these examples are merelyillustrative and are in no way to be interpreted as limiting the scopeof the invention.

EXAMPLES Comparative Example Formation of an Optical Polarizer with aPolyvinyl Alcohol Dichroic Polarizing Film.

A solution containing 10 wt. % Airvol 107 polyvinyl alcohol (AirProducts, Allentown, Pa.) and 0.1 wt. % Triton X-100 (Union Carbide,Danbury, Conn.) was coated onto a corona treated unoriented polyestercast web having four stacks of 209 optical layers, each utilizing a shoecoater which delivered a wet coating thickness of 64 μm (2.50 mils) ofthe solution. The coating was dried at 105° C. for 1 minute. Themultilayer polyester cast web and the polyvinyl alcohol film wereoriented simultaneously at 156° C. in a tenter oven in the directiontransverse to the direction of extrusion of the reflective polarizer.The reflective polarizer and polyvinyl alcohol film were stretched to 6times their original width.

The polyvinyl alcohol film was stained in an aqueous iodine/potassiumiodide solution at 35° C. for 20 sec. The staining solution contained0.4 wt. % iodine and 21 wt. % potassium iodide. The stain was fixed in abath of boric acid/borax at 65° C. for 25 sec. The fixing solutioncontained 4.5 wt. % borax and 14.5 wt. % boric acid.

The optical polarizer transmitted 83.5% of light having the desiredpolarization and had a Q value of 17. The parameter Q is sometimesreferred to as the dichroic ratio. This ratio, Q, is expressed in termsof the power transmissions of the high transmission polarization stateand the extinction polarization state as follows:

 Q=ln(T _(ext))/ln(T _(trans))

Where T_(trans) is the transmission of the high transmission state andT_(ext) is the transmission of the extinction state.

The polyvinyl alcohol film showed severe cracking and failed in a crosshatched tape pull adhesion test. The crosshatched tape pull adhesiontest was performed as follows. First, a sample was placed on a clean,hard surface. Then, using a plastic template with a ⅛″ slot spaced every¼″, the sample was scribed with a scribing tool to produce acrosshatched pattern. The scribe went through the coating to thesubstrate without going through the substrate. A 4″ strip of 1″ wideScotch Brand #610 (3M Co., St. Paul, Minn.) tape was placed on thediagonal to the crosshatched pattern. Using the template, the tape waspressed down firmly to the sample. Next, using one swift movement, thetape was stripped off the sample at a low angle to the sample surfaceand in a direction away from the operator's body. The sample wasexamined for coating removal. If no coating has been removed, the samplepasses the test. If any coating has been removed from the sample, thesample failed the test. In this particular case, the sample failed thetest.

Example 1 Formation of an Optical Polarizer with a Polyvinyl AlcoholDichroic Polarizing Film Containing a Sulfonated Polyester.

An aqueous dispersion containing 9 wt. % Airvol 107 polyvinyl alcohol(Air Products, Allentown, Pa.), 1 wt. % WB54 (a sulfonated polyesterfrom 3M Co., St. Paul, Minn.), 3 wt. % N-methylpyrrolidone (availablefrom Aldrich, Milwaukee, Wis.) and 0.1 wt. % Triton X100(Union Carbide,Danbury, Conn.) was coated onto a corona treated unoriented multilayerpolyester cast web, having four stacks of 209 optical layers each, usinga shoe coater which delivered a wet coating thickness of 64 μm (2.50mils) of the polyvinyl alcohol dispersion. The coating was dried at 105°C. for 1 minute. The polyvinyl alcohol coating and the multilayer castweb were preheated in a tenter oven zone heated with hot air charged to160° C. and then drawn to six times their original width over 35 secondsin a tenter zone heated with hot air charged at 150° C. The films werethen heated an additional 85 seconds prior to cooling. The constructionexhibited only a low level of isolated, non-specific defects, possiblydue to impurities or air bubbles.

The polyvinyl alcohol film was stained using an aqueous iodine/potassiumiodide solution at 35° C. for 20 sec. The staining solution contained0.4 wt. % iodine and 21 wt. % potassium iodide. The stain was fixed in abath of boric acid/borax at 65° C. for 25 sec. The fixing solutioncontained 4.5 wt. % borax and 14.5 wt. % boric acid.

The optical polarizer transmitted 87.0% of light having the desiredpolarization and had a dichroic ratio, Q, of 17. The substrate passedthe crosshatched tape pull adhesion test.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

We claim:
 1. A method of making a polarizing film, the methodcomprising: forming a polymeric film on a substrate, the polymeric filmcomprising polyvinyl alcohol and a water-soluble or water-dispersiblesecond polymer in a ratio of polyvinyl alcohol:second polymer in therange of 5:1 to 100:1 by weight; orienting the polymeric film whiledisposed on the substrate; incorporating dichroic dye material into thepolymeric film; and removing the polymeric film from the substrate. 2.The method of claim 1, wherein incorporating dichroic dye material intothe polymeric film occurs prior to removing the polymeric film from thesubstrate.
 3. The method of claim 1, wherein incorporating dichroic dyematerial into the polymeric film occurs after orienting the polymericfilm.
 4. The method of claim 1, wherein incorporating dichroic dyematerial into the polymeric film occurs before orienting the polymericfilm.
 5. The method of claim 1, further comprising disposing thepolymeric film on a second substrate after removing the polymeric filmfrom the substrate.
 6. The method of claim 5, wherein disposing thepolymeric film on a second substrate comprises adhering the polymericfilm to the second substrate.
 7. The method of claim 5, wherein thesubstrate comprises a reflective polarizer.
 8. The method of claim 5,wherein the substrate comprises a mirror.
 9. The method of claim 1,wherein orienting the polymeric film comprises stretching the polymericfilm in at least one direction.
 10. The method of claim 9, whereinorienting the polymeric film comprises stretching the polymeric film andthe substrate in at least one direction.
 11. A method of making adisplay device, the method comprising: forming a polymeric film on asubstrate, the polymeric film comprising polyvinyl alcohol and awater-soluble or water-dispersible second polymer in a ratio ofpolyvinyl alcohol: second polymer in the range of 5:1 to 100:1 byweight; orienting the polymeric film while on the substrate;incorporating dichroic dye material into the polymeric film to form apolarizing film; removing the polymeric film from the substrate; andpositioning the polarizing film in the display device.
 12. The method ofclaim 11, further comprising disposing the polymeric film on a secondsubstrate after removing the polymeric film from the substrate.
 13. Themethod of claim 12, wherein disposing the polymeric film on a secondsubstrate comprises adhering the polymeric film to the second substrate.14. The method of claim 12, wherein the substrate comprises a reflectivepolarizer.
 15. The method of claim 12, wherein the substrate comprises amirror.
 16. A method of making an optical device, the method comprising:forming a polymeric film on a substrate, the polymeric film comprisingpolyvinyl alcohol and a water-soluble or water-dispersible secondpolymer in a ratio of polyvinyl alcohol:second polymer in the range of5:1 to 100:1 by weight; orienting the polymeric film while on thesubstrate; incorporating dichroic dye material into the polymeric filmto form a polarizing film; removing the polymeric film from thesubstrate; and positioning the polarizing film in the optical device.17. The method of claim 16, further comprising disposing the polymericfilm on a second substrate after removing the polymeric film from thesubstrate.
 18. The method of claim 17, wherein disposing the polymericfilm on a second substrate comprises adhering the polymeric film to thesecond substrate.
 19. The method of claim 17, wherein the substratecomprises a reflective polarizer.
 20. The method of claim 17, whereinthe substrate comprises a mirror.
 21. The method of claim 1, wherein thesecond polymer is selected from the group consisting of polyvinylpyrrolidone and polyesters.
 22. The method of claim 11, wherein thesecond polymer is selected from the group consisting of polyvinylpyrrolidone and polyesters.
 23. The method of claim 16, wherein thesecond polymer is selected from the group consisting of polyvinylpyrrolidone and polyesters.